Mass Transfer in Chemical Engineering Processes Part 3 docx

In packed columns, mass transfer efficiency is related to intimate contact and rate transfer between liquid and vapor phases.. According to the double film theory, HETP can be evaluated

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HETP Evaluation of Structured and Randomic Packing Distillation Column

Marisa Fernandes Mendes

Chemical Engineering Department, Technology Institute,

Universidade Federal Rural do Rio de Janeiro

Brazil

1 Introduction

Packed columns are equipment commonly found in absorption, distillation, stripping, heat exchangers and other operations, like removal of dust, mist and odors and for other purposes Mass transfer between phases is promoted by their intimate contact through all the extent of the packed bed The main factors involving the design of packed columns are mechanics and equipment efficiency Among the mechanical factors one could mention liquid distributors, supports, pressure drop and capacity of the column The factors related

to column efficiency are liquid distribution and redistribution, in order to obtain the maximum area possible for liquid and vapor contact (Caldas and Lacerda, 1988)

These columns are useful devices in the mass transfer and are available in various construction materials such as metal, plastic, porcelain, ceramic and so on They also have good efficiency and capacity, moreover, are usually cheaper than other devices of mass transfer (Eckert, 1975)

The main desirable requirements for the packing of distillation columns are: to promote a uniform distribution of gas and liquid, have large surface area (for greater contact between the liquid and vapor phase) and have an open structure, providing a low resistance to the gas flow Packed columns are manufactured so they are able to gather, leaving small gaps without covering each other Many types and shapes of packing can satisfactorily meet these requirements (Henley and Seader, 1981)

The packing are divided in random – randomly distributed in the interior of the column – and structured – distributed in a regular geometry There are some rules which should be followed when designing a packed column (Caldas and Lacerda, 1988):

a The column should operate in the loading region (40 to 80% flooding), which will assure the best surface area for the maximum mass transfer efficiency;

b The packing size (random) should not be greater than 1/8 the column diameter;

c The packing bed is limited to 6D (Raschig rings or sells) or 12D for Pall rings It is not recommended bed sections grater than 10m;

d Liquid initial distribution and its redistribution at the top of each section are very important to correct liquid migration to the column walls

A preliminary design of a packed column involves the following steps:

1 Choice of packing;

2 Column diameter estimation;

3 Mass transfer coefficients determination;

4 Pressure drop estimation;

5 Internals design

This chapter deals with column packing efficiency, considering the main studies including

random and structured packing columns In packed columns, mass transfer efficiency is

related to intimate contact and rate transfer between liquid and vapor phases The most

used concept to evaluate the height of a packed column, which is related to separation

efficiency, is the HETP (Height Equivalent to Theoretical Plate), defined by the following

equation:

   

in which Z is the height of the packed bed necessary to obtain a separation equivalent to N

theoretical stages (Caldas and Lacerda, 1988)

Unfortunately, there are only a few generalized methods available in the open literature for

estimating the HETP These methods are empirical and supported by the vendor advice The

performance data published by universities are often obtained using small columns and with

packing not industrially important When commercial-scale data are published, they usually

are not supported by analysis or generalization (Vital et al., 1984) Several correlations and

empirical rules have been developed for HETP estimation in the last 50 years Among the

empirical methods, there is a rule of thumb for traditional random packing that says

That rule can be used only in small diameter columns (Caldas and Lacerda, 1988)

The empirical correlation of Murch (1953) cited by Caldas and Lacerda (1988) is based on

HETP values published for towers smaller than 0.3 m of diameter and, in most cases,

smaller than 0.2 m The author had additional data for towers of 0.36, 0.46 and 0.76 m of

diameter The final correlation is

K1, K2 and K3 are constants that depend on the size and type of the packing

Lockett (1998) has proposed a correlation to estimate HETP in columns containing

structured packing elements It was inspired on Bravo et al.’s correlation (1985) in order to

develop an empirical relation between HETP and the packing surface area, operating at 80%

flooding condition (Caldas and Lacerda, 1988):

According to the double film theory, HETP can be evaluated more accurately by the

following expression (Wang et al., 2005):

Therefore, the precision to evaluate HETP by equation (6) depends on the accuracy of

correlations used to predict the effective interfacial area and the vapor and liquid mass

transfer coefficients So, we shall continue this discussion presenting the most used

correlations for wetted area estimation, both for random and structured packed columns

Wang et al (2005) also presented a complete discussion about the different correlations

mostly used for random and structured packing

2 Literature review

The literature review will be divided in two sections, treating and analyzing separately

random and structured distillation columns as the correlations for the effective area and

HETP evaluation

2.1 Part A: performance of random packing

Before 1915, packed columns were filled with coal or randomly with ceramic or glass shards

This year, Fredrick Raschig introduced a degree of standardization in the industry Raschig

rings, together with the Berl saddles, were the packing commonly used until 1965 In the

following decade, Pall rings and some more exotic form of saddles has gained greater

importance (Henley and Seader, 1981) Pall rings are essentially Raschig rings, in which

openings and grooves were made on the surface of the ring to increase the free area and

improve the distribution of the liquid Berl saddles were developed to overcome the Raschig

rings in the distribution of the liquid Intalox saddles can be considered as an improvement

of Berl saddles, and facilitated its manufacture by its shape The packing Hypac and Super

Intalox can be considered an improvement of Pall rings and saddles Intalox, respectively

(Sinnott, 1999) In Figure 1, the packing are illustrated and commented

The packing can be grouped into generations that are related to the technological advances

The improvements cited are from the second generation of packing Today, there are

packing of the fourth generation, as the Raschig super ring (Darakchiev & Semkov, 2008)

Tests with the objective to compare packing are not universally significant This is because

the efficiency of the packing does not depend, exclusively, on their shape and material, but

other variables, like the system to be distilled This means, for example, that a packing can

not be effective for viscous systems, but has a high efficiency for non-viscous systems

Moreover, the ratio of liquid-vapor flow and other hydrodynamic variables also must be

considered in comparisons between packing The technical data, evaluated on packing, are,

generally, the physical properties (surface area, free area, tensile strength, temperature and

chemical stability), the hydrodynamic characteristics (pressure drop and flow rate

allowable) and process efficiency (Henley and Seader, 1981) This means that Raschig rings

can be as efficient as Pall rings, depending on the upward velocity of the gas inside the

column, for example These and other features involving the packing are extensively

detailed in the study of Eckert (1970)

Ref: Henley & Seader (1981)

Fig 1 Random packing: (a) plastic pall rings (b) metal pall rings (Metal Hypac) (c) Raschig

rings (d) Intalox saddles (e) Intalox saddles of plastic (f) Intalox saddles

In literature, some studies on distillation show a comparison between various types of

random and structured packing Although these studies might reveal some tendency of the

packing efficiency for different types and materials, it is important to emphasize that they

should not generalize the comparisons

Cornell et al (1960) published the first general model for mass transfer in packed columns

Different correlations of published data of H L and H V, together with new data on industrial

scale distillation columns, were presented to traditional packing, such as Raschig rings and

Berl saddles, made of ceramic Data obtained from the experimental study of H L and H V

were analyzed and correlated in order to project packed columns The heights of mass

transfer for vapor and liquid phases, are given by:

H   C   S

which:

0 16 1

1 25 2

In the f factors, the liquid properties are done in the same conditions of the column and the

water properties are used at 20 ºC The parameters n and m referred to the packing type,

being 0.6 and 1.24, respectively, for the Raschig rings C fL represents the approximation

coefficient of the flooding point for the liquid phase mass transfer The values of φ and Ψ are

packing parameters for the liquid and vapor phase mass transfer, respectively, and are

graphically obtained In this correlation, some variables don’t obey a single unit system and

therefore need to be specified: dc(in), Z(ft), H(ft), G(lbm/h.ft2)

Onda et al (1968 a, b) presented a new model to predict the global mass transfer unit In

this method, the transfer units are expressed by the liquid and vapor mass transfer

where Γ is a constant whose values can vary from 5.23 (normally used) or 2, if the packing

are Raschig rings or Berl saddles with dimension or nominal size inferior to 15 mm

It can be noted, in these equations, the dependence of the mass transfer units with the wet

superficial area It is considered, in this model, that the wet area is equal to the liquid-gas

interfacial area that can be written as

G We



2 2

L L

a G Fr

The equation for the superficial area mentioned can be applied, with deviations of,

approximately, 20% for columns packed with Raschig rings, Berl saddles, spheres, made of

ceramic, glass, certain polymers and coatted with paraffin

Bolles and Fair (1982) compiled and analyzed a large amount of performance data in the

literature of packed beds, and developed a model of mass transfer in packed column

Indeed, the authors expanded the database of Cornell et al (1960) and adapted the model to

new experimental results, measured at larger scales of operation in another type of packing

(Pall rings) and other material (metal) The database covers distillation results in a wide

range of operating conditions, such as pressures from 0.97 to 315 psia and column diameters

between 0.82 to 4.0 ft With the inclusion of new data, adjustments were needed in the

original model and the values of φ and Ψ had to be recalculated However, the equation of

Bolles and Fair model (1982) is written in the same way that the model of Cornell et al

(1960) The only difference occurs in the equation for the height of mass transfer to the vapor

phase, just by changing the units of some variables:

In this equation, d’ C is the adjusted column diameter, which is the same diameter or 2 ft, if

the column presents a diameter higher than that

Unlike the graphs for estimating the values of φ and Ψ, provided by Cornell et al (1960),

where only one type of material is analyzed (ceramic) and the percentage of flooding,

required to read the parameters, is said to be less than 50% in the work of Bolles and Fair

(1982), these graphics are more comprehensive, firstly because they include graphics for

Raschig rings, Berl saddles and metal Pall rings, and second because they allow variable

readings for different flooding values

The flooding factor, necessary to calculate the height of a mass transfer unit in the Bolles and

Fair (1982) model, is nothing more than the relation between the vapor velocity, based on

the superficial area of the column, and the vapor velocity, based on the superficial area of

the column at the flooding point The Eckert model (1970) is used for the determination of

these values The authors compared the modified correlation with the original model and

with the correlation of Onda et al (1968 a, b), concluding that the lower deviations were

obtained by the proposed model, followed by the Cornell et al (1960) model and by the

Onda et al (1968 a, b) model

Bravo and Fair (1982) had as objective the development of a general project model to be

applied in packed distillation columns, using a correlation that don’t need validation for the

different types and sizes of packing Moreover, the authors didn’t want the dependence on

the flooding point, as the model of Bolles and Fair (1982) For this purpose, the authors used

the Onda et al (1968 a, b) model, with the database of Bolles and Fair (1982) to give a better

correlation, based on the effective interfacial area to calculate the mass transfer rate The

authors suggested the following equation:

Evidently, the selection of k V e k L models is crucial, being chosen by the authors the models

of Shulman et al (1955) and Onda et al (1968a, b), since they correspond to features

commonly accepted The latter equation has been written in equations 23 and 24 For the

first, we have:

0 36

2 3

The database used provided the necessary variables for the effective area calculation by the

both methods These areas were compared with the known values of the specific areas of the

packing used Because of that, the Onda et al (1968 a, b) model was chosen to provide

moderate areas values, beyond cover a large range of type and size packing and tested

systems

The authors defined the main points that should be taken in consideration by the new

model and tested various dimensional groups, including column, packing and systems

characteristics and the hydrodynamic of the process The better correlation, for all the

systems and packing tested is given by:

Recently, with the emergence of more modern packing, other correlations to predict the rate

of mass transfer in packed columns have been studied Wagner et al (1997), for example,

developed a semi-empirical model, taking into account the effects of pressure drop and

holdup in the column for the Nutter rings and IMTP, CMR and Flaximax packing These

packing have higher efficiency and therefore have become more popular for new projects of

packed columns today However, for the traditional packing, according to the author, only

correlations of Cornell et al (1960), Onda et al (1968a, b), Bolles and Fair (1982) and Bravo

and Fair (1982), presented have been large and viable enough to receive credit on

commercial projects for both applications to distillation and absorption

Berg et al (1984) questioned whether the extractive distillation could be performed in a packed

distillation column, or only columns with trays could play such a process Four different

packing were used and ten separation agents were applied in the separation of ethyl acetate

from a mixture of water and ethanol, which results in a mixture that has three binary

azeotropes and a ternary A serie of runs was made in a column of six glass plates, with a

diameter of 3.8 cm, and in two packed columns Columns with Berl saddles and Intalox

saddles (both porcelain and 1.27 cm) had 61 cm long and 2.9 cm in diameter The columns

with propellers made of Pyrex glass and with a size of 0.7 cm, and Raschig rings made of flint

glass and size 0.6 cm, were 22.9 cm long and 1.9 cm in diameter The real trays in each column

were determined with a mixture of ethyl benzene and m-xylene The cell, fed with the mixture,

remained under total reflux at the bubble point for an hour After, the feed pump was

switched on and the separating agent was fed at 90 °C at the top of the column Samples from

the top and bottom were analyzed every half hour, even remain constant, two hours or less

The results showed, on average, than the packed column was not efficient as the columns of

plates for this system The best packing for this study were, in ascending order, glass helices,

Berl saddles, Intalox saddles and Raschig rings The columns with sieve plates showed the best

results Propellers glass and Berl saddles were not as effective as the number of perforated

plates and Intalox saddles and Raschig rings were the worst packing tested When the

separating agent was 1,5-pentanediol, the tray column showed a relative volatility of ethyl

acetate/ethanol of 3.19 While the packed column showed 2.32 to Propeller glass, 2.08 for Berl

saddles, 2.02 for Intalox saddles and 2.08 for Raschig rings

Through the years, several empirical rules have been proposed to estimate the packing

efficiency Most of the correlations and rules are developed for handles and saddle packing

Vital et al (1984) cited several authors who proposed to develop empirical correlations for

predicting the efficiency of packed columns (Furnas & Taylor, 1940; Robinson & Gilliand,

1950; Hands & Whitt, 1951; Murch, 1953; Ellis, 1953 and Garner, 1956)

According to Wagner et al (1997), the HETP is widely used to characterize the ability of

mass transfer in packed column However, it is theoretically grounded in what concerns the

mass transport between phases Conversely, the height of a global mass transfer, HOV, is

more appropriate, considering the mass transfer coefficient (k) of the liquid phase

(represented by subscript L) and vapor (represented by subscript V) individually Thus, the

knowledge of the theory allows the representation:

V V

G H

k a P M



L L

G H



The effective interfacial mass transfer area, in a given system, is considered equal to the

liquid and vapor phases, as is the area through which mass transfer occurs at the interface It

is important to also note that a e is not composed only by the wet surface area of the packing

(a W), but throughout the area that allows contact between the liquid and vapor phases

(Bravo and Fair, 1982) This area can be smaller than the global interfacial area, due to the

existence of stagnant places, where the liquid reaches saturation and no longer participate in

the mass transfer process Due to this complicated physical configuration, the effective

interfacial area is difficult to measure directly The authors proposed a new model using

high-efficiency random packing as IMTP, CMR, Fleximac and Nutter The final model

After the test using 326 experimental data, the predicted values of HETP showed a deviation

less than 25% from the experimental results It was observed that physical properties have a

little effect on mass transfer

Four binary systems were tested (cyclohexane-heptane, methanol-ethanol,

ethylbenzene-styrene and ethanol-water) from different database and different packing types and sizes

The only packing parameter needed was a packing characteristic which has a value of 0.030

for a 2 in Pall and Raschig rings and about 0.050 for 2 in nominal size of the high efficiency

packing investigated

The theoretical relations between the mass transfer coefficient and a packing efficiency

definition, are not easily obtained, in a general manner This is due to the divergence

between the mechanisms of mass transfer in a packed section and the concept of an ideal

stage The theoretical relation deduced, applied in the most simple and commonly situation

Although validated only for the cases of dilute solutions, constant inclination of the

equilibrium line, constant molar flow rates, binary systems and equimolar countercurrent

diffusion, this equation has been applied to systems with very different conditions from

these, and even for multicomponent systems (Caldas & Lacerda, 1988)

The design of packed columns by the method of the height of a global mass transfer unit is

an established practice and advisable For this, it is necessary to know the height of the mass

transfer unit for both liquid and for vapor phases

H L values are usually experimentally obtained by absorption and desorption of a gas,

slightly soluble, from a liquid film flowing over a packed tower, in a countercurrent mode

with an air stream Under these conditions, changes in gas concentration are neglected and

no resistance in the gas film is considered The variables that affect the height of the liquid transfer unit are the height of the packed section, gas velocity, column diameter, the physical properties of liquid and the type and size of the packing

The values of the height of a transfer unit of a gas film, H V, need to be measured under the same conditions as the resistance of the liquid film is known This can be done by the

absorption of a highly soluble gas An alternative method to determine H V involves the vaporization of a liquid, at constant temperature, within a gas stream In this case, the

resistance of the liquid film is zero and H V is equal to H OV The variables that affect the height of transfer unit of a gas film are the gas and liquid velocities, the physical properties of the gas, column diameter, the height, type and size of the packing (Cornell et al., 1960)

Linek et al (2001) studied the hydraulic and mass transfer data measuring pressure drop, liquid hold-up, gas and liquid side volumetric mass transfer coefficients and the interfacial area for Rauschert-Metall-Sattel-Rings (RMSR) with 25, 40 and 50 mm The shape and characteristics of the studied packing corresponded with the metal Pall rings and Intalox packing of Norton The distillation experiments were performed using the systems methanol-ethanol, ethanol-water and isooctane-toluene at atmospheric pressure in a column

of diameter 0.1 m and a height of packing 1.67 m, operated under total reflux The measured values of HETP were compared with those calculated for the different sizes of RMSR packing for the distillation systems The calculated values differ by less than ± 15% from the experimental values, with the exception for the data obtained at extremely low gas flow rates in the system ethanol-methanol for which the respective difference reached 46% Figure 2, from the paper of Linek et al (2001), shows the comparison of measured values of HETP with those calculated from absorption mass transfer data using the model described

in Linek et al (1995) cited by Linek et al (2001)

Fig 2 Comparison of measured values of HETP with those calculated from absorption mass transfer data (Linek et al., 2001)